Anti-neutrophil cytoplasmic antibodies (ANCA) are clearly the basis of ANCA glomerulonephritis (GN); however, the fundamental immune defects that lead to pathologic ANCA production are still incompletely understood. This project seeks to understand how dysregulation of the innate and adaptive immune response promotes human ANCA glomerulonephritis. Understanding these fundamental mechanisms underlying the development of ANCA will allow for the development of mechanism-based therapies and potentially antigen- specific tolerance regimens to avert disease onset (prevention) and relapse. This project will take advantage of a longitudinal ANCA disease cohort, as well as the Department of Defense repository which includes sera obtained before disease onset, to define these pathogenic mechanisms. Aim 1 will utilize mass spectrometry methods to decipher the epitope specificity of patients' autoantibodies before disease onset, during disease remission and relapse, with the ultimate goal of defining the inciting, or at least immunodominant, epitope of disease onset and relapse. Knowledge and understanding of such epitopes will provide a basis for antigen- specific prevention of disease. Aim 2 builds upon the recent identification of a distinct population of T cells that is pro-inflammatory and resistant to regulation in ANCA GN patients. These studies will fully interrogate these cells' transcriptomic profile, autoreactive nature and genetic underpinnings. The potential for targeting this population with antibody-based therapies will also be determined. In Aim 3, a population of CD33+ myeloid cells with potential capacity for T cell immunoregulation will be explored. This cell population is unexpectedly expanded in ANCA GN, have a ?granulocytic? phenotype, and express ANCA antigens (MPO and PR3). Is it possible that ANCA bind CD33+ myeloid cells and subvert their suppressive potential? These objectives, when taken together, begin to answer one of the most common questions pertaining to autoimmune diseases: What causes or incites the disease? Results from the proposed objectives can inform; 1) inciting autoantigenic epitopes?are they a consequence of environmental exposures or infections?; 2) what is the phenotype of autoreactive T cells?can these cells be removed from circulation?; 3) what is the underlying genetic and transcriptomic profile of a specific pro-inflammatory T cell population?; 4) how does altered glycation of autoantibodies affect binding to downstream immunologic targets?; and 5) how do ANCA functionally alter the CD33+ myeloid population? Answers to these questions will provide the needed insight and tools to enforce immune tolerance, to provide biomarkers for disease activity and to inform predisposing factors related to disease development. Not only can these studies alter the current knowledge and treatment of ANCA glomerulonephritis, but have far-reaching impacts to numerous other autoimmune diseases.